Inspired by this thread:
http://boards.straightdope.com/sdmb/showthread.php?t=837357
What was the purpose of the old multi-tube jet exhausts of yesteryear’s airliners?
They are noise suppressors and work by mixing the exhaust stream. I suppose they are not so much a feature of old engines but a consequence of combining loud engines with populated areas (old military aircraft didn’t have noise suppression). More modern engines with high bypass ratios are much quieter and don’t need the suppression on the nozzle itself.
https://engineering.purdue.edu/~propulsi/propulsion/jets/basics/noise.html
what Richard Pearse said. I believe they’re called “hush kits” if they’re retrofits.
See here for more: Hush kit - Wikipedia
Here’s an alternate arrangement: General Electric CJ805 - Wikipedia
That’s it. The idea is that exhaust noise is produced by shear between the high-velocity exhaust and the low-velocity ambient air. It is reduced either by reducing the difference in velocity at the shear surfaces or by adding shear layers, which is what happens with high-bypass engines, or by spreading the shear out over a larger area. The multi-pipe exhaust ducts you see there (apparently on an early 707) are one way but cost performance and weight, daisy mixers (looking like flowers) are another, the chevrons on 787’s (even though that’s a lower-shear surface between the fan and ambient air) are another still.